Master Organic Chemistry Reaction Guide

Addition of HBr to Alkenes

1-Header

Description: Treatment of alkenes with hydrobromic acid will result in the formation of alkyl bromides.

Notes: This is an addition reaction. Note that the bromine always ends up at the more substituted carbon of the alkene (Markovnikoff-selectivity)

Examples:

2-Examples

Notes: When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. Note the third example – where Markovnikoff’s rule gives no clear preference, a mixture will be obtained. Note that the right hand molecule of the third example contains a stereocenter, which will be obtained as a racemic mixture (1:1 mixture of enantiomers).

Mechanism: Electrons from the C1-C2 π bond attack the hydrogen of HBr, expelling the bromide anion and leading to the formation of a carbocation (Step 1, arrows A and B). Note here that the carbocation preferentially forms on C2 (secondary) and not C1 (primary) since secondary carbocations are more stable. The bromide anion then attacks the carbocation, leading to formation of the alkyl bromide (Step 2, arrow C)

Notes: Since the carbocation is planar (flat) there is no preferred direction of attack of the bromide ion. If it’s possible to form a stereocenter, a mixture of stereoisomers will be obtained.

3-b-mech

Additional example (advanced) : If the molecule contains adjacent stereocenters, the two directions of attack on the carbocation will no longer be of equal energy and a mixture of diastereomers will be obtained. In this example, attack of the bromide ion on the less hindered “top face” of the carbocation is favored, and a mixture of products (diastereomers) will be obtained.

Quiz Yourself!

What is the product of this reaction?

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What alkene would give the following product?

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Either one of these alkenes would work.

Further Reading:

  1. First example
    The Stereochemistry of the Addition of Hydrogen Bromide to 1,2-Dimethylcyclohexene
    George S. Hammond and Thomas D. Nevitt
    Journal of the American Chemical Society 1954 76 (16), 4121-4123
    DOI:1021/ja01645a020
    Early paper from the 50’s by Prof. George Hammond (of Hammond’s Postulate) on the mechanism of HBr addition to 1,2-dimethylcyclohexane. He prefers a concerted pathway, although that might due to the conditions he employs – in pentane, a very nonpolar solvent, polar intermediates are disfavored.
  2. Mechanistic studies
    Hydrochlorination of cyclohexene in acetic acid. Kinetic and product studies
    Robert C. Fahey, Michael W. Monahan, and C. Allen McPherson
    Journal of the American Chemical Society 1970 92 (9), 2810-2815
    DOI:1021/ja00712a034
    Detailed kinetic studies of the addition of HCl to cyclohexene in acetic acid, discussing a possible third-order mechanism (rate = k[cyclohexene][HX]2).
  3. Experimental Procedure
    Lee, T. V.; Porter, J. R. Org. Synth. 1995, 72, 189
    DOI: 10.15227/orgsyn.072.0189
    The first reaction in the above procedure involves two steps – addition of HBr across the double bond and converting the aldehyde to a dimethyl acetal.

 

Comments

Comment section

15 thoughts on “Addition of HBr to Alkenes

  1. This really is as good as anything out there- and I think the way it is neatly packaged makes it better than the rest. I am new as of yesterday to being a member- so if this is a frequently asked new kid question I apologize- but is there a clean way to print these? I filled out the mechanism blank sheets in Organic Chem as a Second Language and I wanted to print these out so that I can study from both my copy of notes and the reactions from this guide to make sure I have it all down- but printing it from a source like this takes a lot of pages since it prints out everything not just the text and pics about the mechanism (like this one short reaction printed is 9 pages). If not- that is okay- thanks for all of your hard work.

    1. Good question! The transition state involves partial bonds and non-ideal bonding geometries, which is less stable than the carbocation itself.

  2. This is not 100% related to this video, but on the topic of alkenes in general. Why is 3-methyl-2-butene more reactive than 2-methyl-1-butene?

  3. Will solvent change the product? Eg.- If water is used as a solvent will OH- attach instead of Br- as it is better nucleophile?

  4. Hello sir, useful information.
    Can you please tell me, if there is option between a OH group and double bond, HBr will add to double bond or protonate OH group?

    1. Proton transfers to OH are generally faster than is addition to a double bond. However, it’s an equilibrium situation. If your molecule contains both an alcohol and an alkene, I’d double-check to make sure that you can’t have intramolecular formation of a 5 or 6 membered ring from 1) addition of HBr across the double bond, and 2) attack of the OH on the resulting carbocation.
      The other option is that the protonated OH leaves as OH2, resulting in a new carbocation, which is then attacked by a double bond as nucleophile. It really depends on your molecule.

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